The answer is 0.
As
6 * 7 = 42 but the answer is written as 2
3 * 9 = 27 but the answer is written as 7
8 * 2 = 16 but the answer is written as 6
If we look at the answers, the second digit has been used in the problem.
Now, as
4 * 5 = 20 and the second digit is 0, the answer is 0.